I had this problem the last two days – out of the blue, the MM2 was running for over a year with no problem. I know the thread is old, but it pops up when searching here or on google for this error, so I wanted to add how I was able to solve it:

I backed up the config, my modules and my custom css.
Then I deleted the MagicMirror-Folder, keeping pm2 and everything unchanged. I cloned MM2, ran “npm install”, brought back the config, modules and custom css and ran “pm2 start mm”.

Seems so easy now, but for two days I tried everything – except deleting the folder.